1. Field of the Invention
The present invention relates to a diaphragm device for a camera and, more particularly, to a diaphragm device having high precision which is suitable for use in a still video camera.
2. Description of the Related Art
In a diaphragm device for a camera, it has been a general practice to adjust an opening diameter when the diaphragm is in its full open state. The conventional diaphragm device is so constructed that the adjustment of the opening diameter is not effected or cannot be effected when the diaphragm is in its reduced opening state. On the other hand, the diaphragm device involves various manufacturing errors (for example, error in shape of diaphragm cam groove, error in position of dowel formed on diaphragm blade, error in position of hole formed in diaphragm rotating ring, etc.), and these errors become larger as the diameter of the diaphragm opening becomes smaller. This is the reason why the number of steps of the diaphragm is not proportional to a quantity of light passing through the diaphragm at the reduced opening side. The error caused at the reduced opening side appears in the usual 35 mm film camera but such error has not presented a severe problem in case of the 35 mm film camera. This is because a photographed picture has a relatively large area and, consequently, the diaphragm opening is relatively large, so that an error is not likely to occur and further because an allowance of illumination of the image plane on the film (in other words, latitude) is relatively large.
In case of a still video camera, however, the state of things is completely different from that of the 35 mm film camera. An image pickup element of the still video camera, which corresponds to the film, has a considerably smaller size than that of the picture of a 35 mm film camera. Since the ratio of focal distances of lenses having the same angle of view is proportional to that of sizes of diagonals of picture planes, an opening diameter of a diaphragm of the still video camera having smaller focal distance is smaller than that of the 35 mm film camera even if they have a same F-number. Accordingly the error at the reduced opening diameter of the diaphragm becomes larger. Furthermore, there is such disadvantage that the image pickup element has a narrower latitude than that of the silver-halide film, so that more precise illumination of an image plane is required in case of the image pickup element. Accordingly, the conventional diaphragm device has a limited performance particularly when it is in a reduced opening state.